1. Field of Use
This invention relates generally to an optical interconnect for providing extremely high interconnectibility among very large scale integration (VLSI) microelectronic components and systems.
2. Description of the Prior Art
With decreasing feature sizes and increasing chip sizes in VLSI technology, the ability to interconnect VLSI chips and the boards that carry them is rapidly becoming an industry concern. As a result the industry is turning to optics technology. An overview of this area is provided in Goodman et al., Optical Interconnections for VLSI Systems, 72 IEEE Proc. 7 (1984), and in Kostuk et al., Optical Imaging Applied to Microelectronic Chip-to-Chip Interconnections, 24 Applied Optics 17 (1985). One assessment of the state of the art of optical interconnects for VLSI technology is presented by Bergman et al., Holographic Optical Interconnects for VLSI, 25 Optical Eng'g 10, at 1109, FIGS. 1-3. The optical interconnects described by the above authors are directed to lessening present and future difficulties in providing a sufficient number of off-chip interconnections for large integrated circuits (IC), reducing propagation delays, and altogether eliminating the problem of RC time constants, inductive noise between lines, and line capacitance particular to electronic interconnects. These authors recognize that numerous different optical signals are able to propagate through the same spatial volume without interference. As a result, they have commented on the attractiveness of an optical interconnect architecture and specifically one employing holographic technology.
The holographic interconnects developed to meet the above needs, however, have been based on conventional transmission or reflection Bragg holography including surface etched gratings. These types of holograms (and holography in general) are described by R. Collier et al., Optical Holography (1971), and Kogelnik, 48 Bell Syst. Tech. J.M. 2909 (1969) and Weller et al., Analysis of Waveguide Gratings: A comparison of the results of Rouard's method and coupled-mode theory, 4 J.Opt.Soc.Am. 60 (1987) for surface-etched gratings. In conventional holographic interconnect architecture, the waveguide that carries the signal, the input sources such as laser diodes, and the output receivers such as photodiodes, fibers, or other detectors, are not located in the plane of the hologram but opposed thereto. This type of free-space holographic optical interconnect, described in the above-cited literature incorporated by reference herein, has important limitations. First, alignment problems are critical. If the sources and detectors and waveguide are not in exact alignment with the opposing holographic element, performance suffers possibly to the point where the interconnect becomes inoperative. Second, and more importantly, even when functioning properly the conventional Bragg holography interconnect cannot, in theory or practical use, provide the large number of interconnections needed in the typical VLSI system. A discussion of these important limitations is provided in T. Jannson, et al., "Real Time Signal Processing IX," 698 S.P.I.E. Proc. 157 (1986) which is incorporated herein by reference as are each of the other cited references herein. Surfaced etched holographic gratings, in particular, are not capable of providing a large number of interconnections. It is not possible to record in the same volume numerous independent surface-etched gratings (Bragg plane sets) without mutual cross-talk between light waves diffracted by the surface gratings. In the last analysis, prior art interconnects have not kept pace with the growing need of high interconnectibility between VLSI components.